As if Gordon Brown didn't have enough problems, the hard left is appealing to disaffected trade unionists by fielding parliamentary candidates in 40 constituencies in protest against Labour's perceived failure to represent workers properly.

Last night's launch of the Trade Unionist and Socialist Coalition (Tusc) rests on general unhappiness with Labour's track record. But who will benefit on polling day?

It's a myth to believe the nation's 6.5 million trade unionists are instinctive Labour voters, but it's fair to say instinctive Labour voters believe in trade unions and their demand for fair rights for workers.

While the Tories have seized on the BA strike by Unite members to hark back to the 1970s – obviously not in a good way – senior ranking members of unaffiliated unions level the opposite charge at the Labour government.

They say the party is "dead as a political vehicle for the interests of workers and trade unionists and that an alternative is needed".

Brown faced the worst of all worlds after describing the BA strike as "unjustified" and "deplorable": David Cameron urged the prime minister to go all the way by urging members to cross the picket line, while the left shook their heads and said the Labour leader had gone too far by uttering the condemnatory words in the first place.

Cries that the Tusc coalition will split the Labour vote in the areas where they are fielding candidates are likely to fall on deaf ears.

A contact at Tusc points out that the coalition is made up of members who have already parted with Labour. Among the groups involved is the Socialist party, formerly Militant Tendency.

Dave Nellist, a former Labour MP who was a member of Militant, will be standing in Coventry North East, for example. He is currently a Socialist councillor on Coventry council.

Bob Crow, the leader of the Rail and Maritime union (RMT), is one of the union leaders supporting the new party.

Crow has form, having provided left-leaning voters with an alternative to the Labour vote before – like Nellist, he was involved in the No2EU – Yes to Democracy alliance, which polled 153,236 votes in last year's European elections.

Funding for Tusc candidates is coming from coalition organisations and individual union branches such as the RMT.

Some senior officials from the Public and Commercial Union, which has just staged another day of strike action over changes to the civil service compensation scheme, also back the new grouping.

This election coalition challenges the idea that unions are, de facto, in Labour's pocket – a view clearly not shared by some in the unaffiliated sector.

As a footnote, I can report that Richard Balfe, the trade union envoy enlisted by Cameron to court trade unions two years ago, is perfectly comfortable with the Tory leader's call for members to ignore their union's democratic mandate and cross the picket line.

The unfailingly courteous Balfe, once a Labour MEP, tells me he does not feel this undermines his efforts to persuade members that the Tory party has changed vis a vis unions.